Social living is a common practice within the animal kingdom that provides well-known evolutionary advantages. For instance, thanks to multiple eyes on the lookout for food and predators, animals that stay together often have higher rates of survival when compared with loners.

But when Guttridge took lemon sharks out of the wild and put them into captivity—eliminating their need to hunt for food and stay wary of threats—the sharks still made an effort to socialize.

“As humans we associate sociality with whales,” Guttridge said. “You don’t often think a shark would be like that—you expect them to be very territorial and aggressive to each other. [Yet] we never see aggression between individuals” of a lemon shark group.

For his previous research, Guttridge captured 42 juvenile lemon sharks from waters off the Bahamas, placing them in square test pens.

These 10-meter-by-10 meter (33-foot-by-33-foot) confinement areas were divided into two outer compartments and a central area. Researchers ran a series of experiments, placing varying numbers of lemon sharks in the outer pens and one in the center. Lemon sharks were given the opportunity to interact with their species in the compartments or remain alone.(See more shark pictures.)

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A lemon shark is seen from below. Photograph courtesy C.J. Crooks

It turns out thatlemon sharks can actually be friendly, according to the research, published in 2009 in the journal Animal Behaviour.

Researchers found that sharks spent significantly more time in the compartments closest to other sharks, even though there was no survival advantage—meaning they wanted to spend time together, Guttridge said. (Also see “Sharks Travel ‘Superhighways,’, Visit ‘Cafes.'”)

“We had an individual in the middle of the pen, and it could swim anywhere it wanted to, but it would always spend more time on the side with other individuals,” he added. “There wasn’t any predation risk. They weren’t getting any more food. They just had this social attraction.”

Sharks of a Fin Stick Together

But, compared with other species of shark, lemon sharks are selective about their social circles. They’re much more likely to pal around with sharks their own size, said Jean-Sebastien Finger, a biologist at the Bimini field station. This gravitation toward similar body types is common within nature: After all, birds of a feather flock together, so it makes sense that sharks of a fin would form a shiver.

Such a specific attraction is likely a survival technique, Sebastien-Finger added, since sharks of similar sizes eat the same types of food and must avoid the same predators. (Also see “Pictures: Biggest Whale Shark ‘Swarm’ Found.”)

There’s also the risk of cannibalism. For these cartilaginous fish, making friends outside their normal social group could be dangerous, since larger lemon sharks will prey upon their smaller cousins.

“It’s fairly complex, these rules of group behaviors and laws of attraction and avoidance,” behavior ecologist Guttridge said.

Swim With Me, Maybe?

Being picky about your friends is also important for passing on knowledge—you can’t leap with the dolphins if you’re lurking with the bottom feeders, for instance. Lemon sharks learn from observing others, according to the recent research, published in 2012 in the journal Animal Cognition.

Lemon sharks (pictured near the Bahamas) are very sociable. Photograph courtesy C.J. Crooks

For the newer study, scientists trained a group of lemon sharks to hit a target, rewarding their successes with food. These learned sharks were paired with sharks that hadn’t had any instruction.

When asked to perform the task later, the newbie sharks completed more trials with a higher rate of success than the control group. This suggests that sharks can use social interactions as opportunities to learn how to find food, identify predators, and woo mates.

“It’s almost like school for us,” Guttridge said. “You learn how to interact with other people. Maybe these sharks are learning how to interact with each other, and as they get older it helps with reproduction.”

A shark’s idea of courtship might not include a declaration of love on Facebook, but this species doesn’t mature until they’re well into their teenage years—plenty of time to study up, Guttridge adds.

Social Suicide

However, there’s a cost to being social. Lemon sharks that hang out in groups have a higher risk of contracting parasites and diseases. If a friend is constantly sick, the benefits of pairing up for safety reasons don’t necessarily compute.

“Because you have more friends, you have less chance of being killed,” Finger said, but there’s also the possibility of disease.